1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus, and more particularly, to a sheet stacking apparatus capable of aligning sheets stacked on a stacking tray, and an image forming apparatus including the sheet stacking apparatus.
2. Description of the Related Art
In recent years, along with increase in image formation speed, there has been known an image forming apparatus including a sheet stacking apparatus configured to stack a large number of sheets having images formed thereon in an aligned state (see U.S. Pat. No. 6,871,851).
The sheet stacking apparatus disclosed in U.S. Pat. No. 6,871,851 includes a pair of alignment members, and is configured to move the pair of alignment members in a sheet width direction orthogonal to a sheet discharge direction on a stacking tray for stacking sheets thereon, thereby aligning the sheets in the sheet width direction. The sheet stacking apparatus includes a sheet stacking height detecting sensor capable of detecting a top surface position of the sheets on the stacking tray in a sheet stacking height direction. Further, the stacking tray is controlled to be raised and lowered in accordance with a detection result obtained from the sheet stacking height detecting sensor so that the top surface position of the stacked sheets with respect to the sheet discharge portion becomes constant.
In the above-mentioned sheet stacking apparatus, in order to perform the alignment or the like without disturbing the aligned state of the sheets stacked on the stacking tray, one of the alignment members is fixed in a state of abutting against one edge portion of the stacked sheets, and another of the alignment members is brought into abutment against another edge portion of the sheets, to thereby perform the alignment. At the time of such an alignment operation, for example, in a case of forming images on a large number of sheets, a user may take a part of the stacked sheets out from the stacking tray during the image forming job. When the user takes out a part of the stacked sheets, the sheet stacking height detecting sensor detects it and raises the stacking tray to an appropriate position. During a period in which the stacking tray is raised to the appropriate position, however, a gap may be formed between one of the alignment members and the topmost sheet. Under the above-mentioned state, when another of the alignment members is to be brought into abutment against another edge portion of a discharged sheet to align the sheet, the sheet may enter the above-mentioned gap to cause stack misalignment, and in some cases, the sheet may slip out through the gap to drop. This may occur, even in a case where both of the pair of alignment members are moved to align the sheets, for example, when the sheet is discharged while being displaced toward one of the alignment members.
Further, for example, in a case where the sheet is discharged with its leading and trailing edges curved upward, that is, in a concave gutter curl state, when the sheet stacking height detecting sensor detects the trailing edge of the sheet that is curved upward, a position higher than the actual top surface position is detected as the top surface position, and as a result, the stacking tray may be lowered. When the stacking tray is lowered in this state, a gap may be formed between the alignment members and the surface of the sheet, and similarly to the above, the stack misalignment may occur in the sheet or the sheet may drop.